O T-shirts to me have always been a lot like the shoe sizes. Very random. In quality and size. I can only assume they come from multiple suppliers and therefore vary in size and quality
From personal experience, I know the contracted manuf. in China put ads in local ExPat magazines looking for "models" for size runs. I had a friend who worked for the company that produces Sean John, Tommy Hilfiger and other brands. So you'd get a college kid who likes baggy clothes show up at the office and the designers ask him "what size do you wear". He LIKES TO BUY XL Shirts and likes the baggy look so he promptly replies, " I wear and Xtra Large". So the line designer happliy measures this kid and uses him as the model for that size run. In reality, this kit is a Large, but based on his answer to the question, the manuf. now believes these dimensions are for an Extra Large.
then you get me showing up for fitting, I say Extra large, and I am, indeed, an extra large. I also used to bring t-shirts made in the US for them to measure, as well. My cousin is a fashion designer who now runs a division of a company that produces clothing for several brands. ALthough he's based in taiwan, he makes it a point to get clothing dimensions from the US, if he knows that line is going to produce clothing for the US market.
Just some more reasons why there coudl be so much disparity..
D